Add getmntent_r to our collection of <mntent.h> compatibility stubs.

This helps build 'external/flo' out of the box.

Bug: 14841211
Change-Id: I30dde77239cceaf1f5743163744eb3604d27a266
diff --git a/libc/include/mntent.h b/libc/include/mntent.h
index 6cc0b18..de285d0 100644
--- a/libc/include/mntent.h
+++ b/libc/include/mntent.h
@@ -35,23 +35,21 @@
 #define MOUNTED _PATH_MOUNTED
 #define MNTTYPE_IGNORE "ignore"
 
-struct mntent
-{
-    char* mnt_fsname;
-    char* mnt_dir;
-    char* mnt_type;
-    char* mnt_opts;
-    int mnt_freq;
-    int mnt_passno;
+struct mntent {
+  char* mnt_fsname;
+  char* mnt_dir;
+  char* mnt_type;
+  char* mnt_opts;
+  int mnt_freq;
+  int mnt_passno;
 };
 
-
 __BEGIN_DECLS
 
-
-struct mntent* getmntent(FILE*);
-FILE* setmntent(const char*, const char*);
 int endmntent(FILE*);
+struct mntent* getmntent(FILE*);
+struct mntent* getmntent_r(FILE*, struct mntent*, char*, int);
+FILE* setmntent(const char*, const char*);
 
 __END_DECLS